SOURCE TESTING ASSOCIATION I Annual Guide 2017


Flexibility can be Key to Getting the Most out of FTIR Gas Analysis


ADVERTORIAL PROVIDED BY PROTEA


Fourier Transform Infra-Red (FTIR) spectroscopy has become an established technology over the last decade in emissions monitoring. When Protea started deploying our analysers to measure process and emissions gases in 1996, the technique was new and we developed the means to get the most out of this powerful technology. We discuss here some details of FTIR hardware, where limitations of the equipment or understanding can result in the benefi ts of FTIR not being realised.


Resolution


The ability of a spectrometer to resolve spectra to the narrowest of widths can be advantageous, especially for identifi cation of overlapping spectral features. If the application requires detection of multiple VOCs species with responses around 2800cm-1


, a higher resolution is a must.


However, in emissions monitoring often the customer knows their gases and there is no need for complex identifi cation. In this case, running at a lower resolution can produce perfectly accurate results. Other benefi ts of low resolution, such as reduced signal-to-noise and quicker measurement times, can be plus points.


By understanding the benefi ts of running at different resolutions the user is able to apply the best choice for any given project. For emissions measurements, a high resolution of 1cm-1


,


gives excellent spectral resolution for gases like HCl or NH3 For process control, switching the analyser to 4cm-1


. for 3sec


measurements, the plant can respond to dynamically changing events. Having fl exibility in resolution enables the most to be made out the FTIR in various applications.


Pathlength


The gas cell pathlength is the distance the IR light passes through the sample. The larger the pathlength, the larger the absorption of light and the IR peak recorded. It is often assumed that the larger the pathlength, the lower the detection limit of concentration. However, with an increase in spectral bounces, there is an increase in loss of light on the detector. This means an increase in signal-to-noise ratio and a higher detection limit. This may have to be rectifi ed with a more complex detector or a longer measurement averaging time to reduce the noise.


In practice gas cell with a short pathlength, such as only 10-20cm, can achieve low detection limits without needing an expensive multipass cell. Likewise a well-aligned analyser with a pathlength of 4m can give as good a performance than a cell with up to 10m. Being fl exible when applying the best cell confi guration will gain the best results.


Scan Rate


Different FTIR designs have different interferometer scan rates. Scans can be produced as quickly as a few milliseconds or as long as a few seconds. Choice of scan rate is a trade-off between measurement noise and speed.


2


atmosFIR EX – An ATEX certifi ed multiple-point, multigas analyser where the fl exibility of FTIR for different measurement is realised in one system


An analyser producing a spectrum every few hundred milliseconds may not be useable at this rate, because the data produced will have too much noise – detection limits are too high. An analyser making a scan every 10 seconds may be too slow to detect the changes in processes.


When specifying the scan rate, one should start from the measurement rate that is required for the project – how often are results to be recorded. Then choosing the scan rate that will give enough data to produce a low noise measurement in that period. Being fl exible to change scan rates when required opens up the deployment of FTIR to more projects.


Sampling Arrangement


As an extractive gas analyser not requiring gas conditioning, FTIR can be quite easy to set-up - a sample probe (fi lter) and sample line before the analyser and a pump (vacuum pump, air eductor) after the analyser. This arrangement can give slightly negative sample pressures, affecting the FTIR spectra. Recording the gas pressure inside the analyser and using a high resolution enable the pressure drifts to be compensated for in real-time.


With low resolution, there can be a non-linear change in the spectra due to pressure, diffi cult to correct for. A sampling arrangement with pump before the analyser can be used. This arrangement does have some advantages, with a faster sampling rate possible, controllable gas cell pressure and the ability to pass sample to multiple analysers.


Being able to sample both with a pump pre-analyser and post- analyser when the project requires it is a useful option available to the user.


Technical Example


This fl exibility of FTIR is demonstrated, in the atmosFIR EX system to measure multiple gas streams across a landfi ll plant. The analyser automatically changes resolution; the process gases are measured at high resolution, to identify all the sub-ppm impurities; the emission gases are measured at lower resolution, giving quicker results for basic emissions gases like CO. Also, longer scans are made from the process samples to reduce noise, quicker scans from the emissions points to sample in the allotted time. The process gases are under positive pressure, so no sample pump is used but the emissions samples use a sample pump after the FTIR analyser. With this fl exibility one FTIR analyser can do the job of multiple analysers in one package.


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